It's worth noting that Mad Magazine has had "marginals" for years, since 1963, actually. They were drawn by Sergio Aragones, also well known for his Groo comics. Here's an excerpt of his wikipedia entry.

Aragonés became a contributor to MAD Magazine in 1963. He was famous for his wordless "drawn-out dramas" or "marginals" appearing in the margins and between panels of the magazine, both horizontally and vertically as well as occasionally around corners. Prior to Aragonés' work for MAD, the marginals had been text jokes. According to Aragonés the staff of MAD enjoyed his marginals, but expected to only last one or two issues. They did not expect him to be able to maintain the steady stream of small cartoons needed for each issue. Aragonés has provided marginals for every issue of MAD since 1963 excepting one [1] (http://www.sergioaragones.com/ask.htm). In addition, Aragones has a feature section in the current issues.